1. Field of the Invention
The invention is directed to epoxy resin systems, particularly low moisture absorption epoxy resin systems. The epoxy resin systems according to the invention have utility in aerospace manufacture, or other applications requiring resin systems having low moisture absorption and good retention of dimensional properties under hot and wet conditions. Prepregs, composites and resin transfer molding applications incorporating the epoxy resin systems of the invention are also disclosed.
2. Description of the Related Art
Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aerospace, automotive, and sporting goods applications. Typically, these composites comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a cured thermosetting resin matrix.
Most advanced composites are fabricated from prepregs, ready-to-mold sheets of fibrous reinforcement impregnated with uncured or partially cured resin. In order to be useful in commercial fabrication operations, prepreg matrix resin needs to have a long xe2x80x9coutlife,xe2x80x9d typically defined as the period of time the prepreg can remain at room temperature and still be useful for making cured composites; that is, the prepreg must remain pliable and retain appropriate tack (stickiness). Pliability is conferred by the resin matrix, which should remain relatively soft and deformable without cracking. Outlife is sometimes referred to herein as xe2x80x9ctack and drape outlife.xe2x80x9d
Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepregs since they possess a balance of properties generally required for such applications. An early resin system extensively used in space applications was based on tetraglycidyl-methylenedianiline [TGMDA] epoxy resin and 4,4xe2x80x2-diaminodiphenylsulfone [4,4xe2x80x2-DDS]. This system has been used extensively in aerospace primary and secondary structures.
As a hardener, DDS has a low level of reactivity with epoxy resins at room temperature, and prepregs made using DDS-based systems have good out-life. The resulting fiber composites have high compressive strength, good fatigue characteristics, and low shrinkage during cure. Most epoxy formulations, including TGMDA, tend to absorb moisture (hygroscopic) which reduces their high temperature properties. Accordingly, there continues to be a need for resin systems and composites having reduced moisture absorption.
Other disadvantages associated with prior art epoxy/carbon fiber prepregs are a tendency toward brittleness and microcracking, and high cure temperature requirements, typically in the neighborhood of 350xc2x0 F.
Typical state-of-the-art resin systems for aerospace applications include polycyanate-based resin systems. These resin systems exhibit relatively low moisture absorption, moderate to high toughness, low microcracking, and low dielectric constant. However, the high cost of polycyanate resins relative to epoxies is a disadvantage. Moreover, polycyanates are sensitive to moisture before cure, which makes special precautions necessary, such as the need to predry core materials to prevent blistering and delaminating during cure or postcure. Further, the laminate surfaces of polycyanate-based resin systems resist bonding, exhibit poor tack and drape and exhibit reduced mechanical outlife and storage life. In general, polycyanate-based resin systems require a 350xc2x0 F. cure temperature. Those polycyanate systems having reduced cure temperatures exhibit especially poor tack and drape, and substantially reduced mechanical life and storage life. It has also been observed that, although initial moisture absorption is low for such systems, the moisture absorption in many cases continues to rise during long term moisture exposure and does not reach equilibrium.
It is an object of the present invention to provide an epoxy resin system having low moisture absorption that avoids the disadvantages and drawbacks associated with prior art resin systems. These resin systems are especially suitable for aerospace applications.
The resin system according to the invention has a first component which is generally formed by reacting dicyclopentadiene, epichlorohydrin and phenol to form a polyglycidyl derivative of a phenol-dicyclopentadiene epoxy polymer, and a second component which is an ortho-alkylated diamine hardener.
The epoxy resin of the instant invention has the following structural formula (I): 
wherein R is hydrogen or halogen, and n is from 0 to about 0.5.
In a more preferred embodiment, R is hydrogen such that the phenol moiety is unsubstituted, and n is equal to about 0.2. An epoxy resin of this embodiment is available from Ciba-Geigy under the tradename TACTIX 556.
In general, the useful hardeners for the epoxy resin of the invention are aromatic hardeners preferably having a benzene skeleton in which substituted alkyl groups(s) are ortho to substituted amine groups(s).
In a more preferred embodiment, the ortho-alkylated aromatic compound has the following formula (II): 
wherein Y is a direct bond, sulfur, oxygen, methyl, substituted methyl, or sulfoxy; R1 and R2 are each C1-C4 straight chain or branched alkyl groups and X is hydrogen, chlorine or bromine.
In a most preferred embodiment, Y is xe2x80x94CH2xe2x80x94, and R1 and R2 are each ethyl. This diamine hardener, 4,4xe2x80x2-methylenebis(2,6-diethylaniline), is available from Lonza Group under the tradename Lonzacure(copyright) M-DEA.
In another preferred embodiment Y is xe2x80x94CH2xe2x80x94, R1 is isopropyl and R2 is methyl. This diamine hardener, 4,4xe2x80x2-methylenebis(2-isopropyl-6-methylaniline), is available from Lonza Group under the tradename Lonzacure(copyright) M-MIPA.
In still another preferred embodiment, Y is xe2x80x94CH2xe2x80x94 and R1 and each R2 are isopropyl. This hardener, 4,4xe2x80x2-methylenebis(2,6-diisopropylaniline), is available from Lonza Group under the tradename Lonzacure(copyright) M-DIPA.
Another group of preferred ortho-alkylated aromatic diamine hardeners is represented by structural formula (III): 
wherein the amine groups are meta- or para- to each other, R3 is C1-C4 branched or straight chain alkyl, R4 and R5 are independently hydrogen, branched or straight-chain alkyl or methylthio.
It has been discovered that epoxy resin systems having the above-described dicyclopentadiene-phenolic skeleton and ortho-alkylated diamine hardeners provide low moisture absorption similar to or better than most polycyanate and other state-of-the-art matrix materials, while at the same time providing a desirable combination of other important properties including: low microcracking after thermal cycling, good tack, drape, mechanical outlife, and storage life. The instant resin system is compatible with ancillary materials used in prepreg production, such as metal-containing catalysts which may be present in the release paper. The resin system forms a low, cured-resin density which enables lower weight structures to be utilized. This feature is especially desirable for weight-critical applications, such as aerospace applications. The resin systems are amenable to standard epoxy processing, yielding high Tg and good mechanical properties and very little change in mechanical properties under hot and wet conditions.
In many instances it is possible to provide the instant resin systems at lower cost than state of the art polycyanate resins.
A further surprising aspect of the resin systems according to the invention is their utility in resin transfer molding (RTM) processes. RTM processes generally require lower viscosity resin systems. As described at greater length hereafter, the low viscosity of the epoxidized dicyclopentadiene-phenol/ortho-alkylated diamine hardener resin system of the invention is an unexpected feature, which makes the resin system particularly suitable for RTM processes.
Therefore, in another aspect, the invention is directed to a resin transfer molding process comprising the steps of (a) transferring a resin system into a closed mold containing a fibrous substrate; (b) impregnating the resin system into the fibrous substrate; and (c) curing the resin-impregnated fibrous substrate in the mold to produce a resin transfer molded product, wherein the resin system comprises an (i) epoxy of formula (I) above and (ii) an ortho-alkylated aromatic diamine hardener.
The resin systems of the invention also find utility in the manufacture of prepregs for making composite materials. Composite materials made by resin transfer molding or of prepreg materials according to the invention have utility in the manufacture of spacecraft structures including, without limitation, satellite buses, solar array structures, antennae, mirrors, and reflectors. The composites of this invention can be used as aircraft parts, such as wing skins, wing-to-body fairings, floor panels, flaps, radomes, or automotive parts, as bumpers and springs, and as pressure vessels, tanks or pipes. Potentially, the resin systems, prepregs and resin transfer molding products can be used in any composite structure where low moisture absorption and retention of mechanical properties under hot and wet conditions would be advantageous, including, without limitation, industrial, commercial or military aircraft manufacture, sporting goods manufacture as golf shafts, tennis racquets and fishing rods, and the like.
In addition to the manufacture of composite structures produced by resin transfer molding (RTM), the resin systems of the present invention find utility in vacuum assisted resin transfer molding (VARTM), resin film infusion (RFI), and wet filament winding processes, where low resin viscosity and long pot life, are important.
The inventive resin systems also have utility in adhesives applications, as supported or unsupported films or pastes. The resin systems may further be useful in electronics applications as encapsulation or potting materials. The resins may be useful in applications requiring low dielectric materials where change of dielectric constant with moisture absorption would adversely affect the application, such as in composite radomes.
The resin systems may find use in discontinuous fiber materials or otherwise reinforced composite materials (molding compounds) for compression, injection, transfer, and bulk or sheet molding process applications.
The epoxy resin used in the resin system of the invention, which is set forth in the above formula (I), is based on an epoxidized reaction product of phenol and dicyclopentadiene. This hydrocarbon backbone structure has an extremely low molecular polarity, and the epoxy resins based on this backbone exhibit very low moisture absorption.
The epoxidized phenol-dicyclopentadiene copolymer has a general formula I as follows: 
wherein R is H or halogen and n is from 0 to about 0.5. More preferably, R is hydrogen and n=0.2.
A variety of such resins are now commercially available having differing molecular weights and include hydrocarbon epoxy novolac resins known as TACTIX 556 and TACTIX 71756 available from Ciba-Geigy; XD-1000, XD-1000-L, and XD-1000-2L, available from Nippon Kayaku; and HP-7200 and HP-7200H, available from DIC. The lowest molecular weight and lowest viscosity TACTIX 556 and XD-1000-2L grades are most preferred for use in advanced composite applications.
While the above epoxy resins are known to the art, nevertheless when they are combined with contemporary hardeners, such as 4,4xe2x80x2-DDS (diaminodiphenyl sulfone) or 3,3xe2x80x2-DDS, high viscosity mixtures with poor tack and drape properties generally result. Accordingly, significant modification of such resin systems with other low viscosity resins is required to enhance tack and drape properties, which further detracts from the low moisture absorption characteristic of such systems. Surprisingly, the combination of TACTIX 556 and the state of the art DDS hardeners results in a resin system absorbing more moisture than systems in which TACTIX 556 is combined with the ortho alkylated aromatic diamine hardeners according to the present invention.
The diamine hardeners used in the present invention are ortho-alkylated aromatic diamines. While the diamines of formulas (II) and (III) are most preferred, other ortho-alkylated diamines are useful. For example, systems comprising a,axe2x80x2-bis (3,5-dimethyl-4-amino)-p-diisopropenylbenzene, formerly available from Shell under the trade name EPON 1062, are expected to yield good resin systems.
A preferred embodiment of the invention involves the use of a resin system containing the epoxy resin of formula (I) above, such as TACTIX 556, and one or more of the dialkylated diamines of formulas (II) or (III). As is known in the art of epoxy resin formulation, the compositions of these mixtures can be varied, resulting in mixtures with varying epoxide-amine hydrogen molar ratios and concomitant physical, chemical and mechanical properties.
In the resin system according to the invention, relative amounts of epoxy resin (epoxide) and ortho-alkylated aromatic diamine components may be expressed in terms of the equivalents of hardener (amine hydrogen) to epoxy resin (epoxide). An equivalent weight of hardener per epoxide of 1.0 occurs when each of the hardener amine hydrogens is replaced with a bond to an epoxide group. For example, in a preferred embodiment, the epoxy resin is TACTIX 556, which has an epoxide equivalent weight of between about 220 and about 240 g/mol, and the amine hardener is LONZACURE(copyright) M-MIPA, having a molecular weight of 310.49 g/mol. An amine hydrogen to epoxide equivalents ratio of 1.0 is present when the weight percentages of the epoxy resin and the amine hardener are about 74.3 wt % and about 25.7 wt %, respectively. In general stoichioemetric ratios of 50%-130% of the theoretical amine-epoxide hydrogen equivalence are preferred, and stoichiometric ratios of 70%-110% are most preferred.
In addition, the cure cycles used to polymerize the resin system can also be varied, which can result in variations in degree of cure and in physical, chemical and mechanical properties. In general, the resin cure times range between about 1.0 hour and 8.0 hours and the cure temperatures range between about 100xc2x0 and 200xc2x0 C. Because of their low viscosity, long gel time and pot-life, these resin systems are ideal for advanced composite part manufacturing such as RTM and RFI.
Continuous fiber-reinforced unidirectional tapes or woven or non-woven fabric prepregs can be readily produced. Further, the low moisture absorption of these systems makes them especially suitable for manufacture of advanced composite structures for spacecraft.
In accordance with the practice of the invention, the resin formulations described above can be further modified with a variety of materials, singly or in combination, to meet the requirements of a particular process or application. For example, a low viscosity epoxide (epoxide modifier) can be used to increase the tack and drape properties of the resin system. Examples of epoxy materials which can be employed in the resin system include, but are not limited to, Bisphenol F epoxides, such as PY306, GY285, or GY281, available from Ciba, or Rutapox 0158 (Bakelite); phenol novolac epoxides, such as DEN 431, available from Dow, or EPON 160, available from Shell; Bisphenol A epoxides such as Epon 825 or Epon 828, from Shell, or DER332 or DER 331 available from Dow; cycloaliphatic epoxides such as CY179, available from Ciba; glycidyl amine epoxides such as triglycidyl 4-aminophenol (available as MY510 from Ciba or Epon 1076 from Shell), TGMDA, available as MY721, MY9655, and MY9663 from Ciba; Tetraglycidyl-4,4xe2x80x2-methylenebis(2-ethylbenzeneamine) available as MY 722 from Ciba; and others known to those of ordinary skill in the art.
The resin system formulations can also be modified with curing catalysts or accelerators to reduce the gel time, flow characteristics, cure temperature, and/or cure time as desired. Suitable types of accelerators include, without limitation, Lewis acid complexes such as boron trifluoride monoethylamine complex (BF3MEA), boron trifluoride piperidine complex (BF3piperidine) available from Atotech USA, and BCl3 complexes available from Ciba; imidazole derivatives such as 2-phenyl-4-methyl-imidazole (Curezol 2P4MZ) or 2-phenylimidazole (Curezol 2PZ) available from Shikoku Chemicals, and the like, dicyandiamide, substituted urea derivatives such as 3-(3,4-dichlorophenyl)-1,1-dimethyl urea available as Diuron from Dupont, acid salts of tertiary amines, salts of trifluoro methane sulfonic acid, organophosphonium halides and the like.
The base resin system formulations can also be modified with a variety of toughening agents known in the art, including, but not limited to thermoplastics, such as poly(arylethersulfones), available, for example, as PES 5003P from Sumitomo; poly(etherimides), available, for example, as Ultem 1000 from General Electric; or poly(imides), available, for example, as Matrimid 5218 or Matrimid 9725 from Ciba. These toughening agents may be dissolved in the uncured resin matrix or present as undissolved filler particles. In addition elastomers such as 1300xc3x9713, 1300xc3x978 1300xc3x9718 CTBN, which are reactive liquid polymers from BF Goodrich may be incorporated into the resin system by simple blending or chemical prereaction with one or more of the epoxy resin components. Preformed elastomeric core-shell types of polymeric particles are useful and are readily available to those skilled in the art.
Other fillers and modifiers may also be incorporated into these systems to impart other desired characteristics to the resin matrix. These include without limitation fumed silica, available as Cabosil M5 or TS720 from Cabot, Aerosil US202 from Degussa, and the like, which can be incorporated to increase the viscosity and reduce the flow of the resin composition during processing and cure; pigments such as carbon black to color the composition; antimony oxide and/or brominated epoxy resins to impart flame retardant properties; and thermally or electrically conductive materials such as BN, Al2O3, silver or aluminum powders to impart thermal and/or electrical conductivity.
A surprising feature of the present invention is that in general the resin systems containing epoxy resin and diamine hardener according to the invention exhibit a lower viscosity than either of the epoxy resin component or the ortho-alkylated diamine hardener component. This xe2x80x9ceutecticxe2x80x9d-like feature is particularly important since, for example, TACTIX 556 itself is a semi-solid at room temperature and would not ordinarily be expected to form a resin system having a sufficiently low viscosity for use in RTM and prepreg applications. However, when combined with the instant diamene hardener, the resulting system exhibits excellent room temperature viscosity which is achieved without the addition of (plasticizing) components that could ultimately increase the moisture absorption of the resin system.
Low moisture absorption is a critical property of resin systems used in advanced composites for space applications. In such applications it is particularly important that the initial resin systems for forming the prepregs and composites retain their desired properties under hot and wet conditions. To measure moisture absorption a 72-hour boiling water weight gain is determined. As used herein 72-hour boiling water weight gain means the amount of moisture taken up by the resin system when the cured resin system is submerged in boiling water and weighed after seventy-two hours. The weight gain is reported as a percentage, relative to the starting weight.
For the neat resin system, which has not been formed into a prepreg by incorporation into reinforcing material, the desired weight gain is less than about 1.3%. A preferred weight gain range is between about 1.2 and 1.3%. In some embodiments, a seventy-two hour boiling water weight gain of less than 1.2% is possible. When low moisture absorption is critical the lower the weight gain, the better.
Another measure of moisture absorption is weight gain at equilibrium in a 50 percent relative humidity environment. To determine such weight gain a cured resin system according to the invention is oven-dried and exposed to a 50 percent relative humidity (RH) ambient atmosphere at room temperature (RT). The system is allowed to reach equilibrium wherein substantially no water is taken up by the resin system over three successive weighings, and the weight gain of the resin is reported as a weight percentage, relative to the starting weight. Neat resin (i.e. resin that has not been impregnated into a fibrous reinforcement material) according to the invention preferably has a 50 percent relative humidity weight gain less than about 1.0%. More preferably, weight gain under these conditions is less than 0.75% and most preferably less than 0.60%. Again the lower the weight gain, the better.
To evaluate the weight gain of prepreg at equilibrium in a 50% relative humidity environment, 2xe2x80x3xc3x972xe2x80x3 laminate samples were machined and predried in an air circulating oven for three to five days at 250xc2x0 F. The dried samples were weighed and placed into a conditioning chamber maintained at 50% relative humidity at room temperature. Equilibrium is defined as constant weight over three successive weighings. A preferred uncured prepreg according to the invention typically has a 50% relative humidity weight gain of less than 0.40%. In a most preferred embodiment, the weight gain under those conditions is typically less than 0.20%. The seventy-two hour boiling water weight gain of the uncured prepregs is generally less than about 0.50%, more preferably less than 0.40%. The percentage weight gain is with respect to the prepreg, including reinforcement.
Prepregs or preimpregnated reinforcement may be prepared by several techniques known in the art, such as wet winding or hot melt. In one method of making impregnated tow or unidirectional tape, fiber is passed into a bath of the epoxy/hardener mixture. Although unnecessary for most applications a non-reactive, volatile solvent such as methyl ethyl ketone may be optionally included in the resin bath to further reduce viscosity. After impregnation, the reinforcement is passed through a die to remove excess resin, sandwiched between plies of release paper, passed through a set of heated rollers, cooled, and taken up on a spool. The resulting prepreg is used within a few days or may be stored for months at 0xc2x0 F. During prepreg manufacture, the resin system typically xe2x80x9cB-stagesxe2x80x9d, or partially advances through the reinforcement.
Composites may be prepared by curing preimpregnated reinforcement using heat and pressure. Vacuum bag/autoclave cures work well with such compositions. Laminates may also be prepared via wet layup followed by compression molding, resin transfer molding, or by resin injection. Typical cure temperatures are 100xc2x0 F. to 500xc2x0 F., preferably 180xc2x0 F. to 450xc2x0 F.
Manufacturing composites typically involves laying up a number of sheets of uncured resin-impregnated fibrous substrates (prepregs) on a suitable tool or mandrel and subjecting them to heat and pressure in order to completely impregnate the sheets. The treated sheets become molded to the configuration of the mold and are then subsequently gelled (or crosslinked). The resin is then completely cured by further heat treatment in order to fix the resulting configuration of the molded laminate.
The resin systems of this invention are well suited for filament winding. In this composite fabrication process, continuous reinforcement in the form of tape or towxe2x80x94either previously impregnated with resin or impregnated during windingxe2x80x94is placed over a rotating and removable form or mandrel in a previously determined pattern. Generally the shape is a surface of revolution and contains end closures. When the proper number of layers are applied, the wound form is cured in an oven or autoclave and the mandrel removed.
In a preferred embodiment, modified or unmodified resin systems are used in conjunction with continuous fiber reinforcements to produce resin-impregnated unidirectional tape or woven fabric (prepreg) materials, which are subsequently used to produce advanced composite parts. In a preferred embodiment, the resin systems comprise between about 30 percent and about 40 percent by weight with respect to the finished, uncured prepreg.
A wide variety of fiber reinforcements are available and can be used in accordance with this invention, including S-glass and E-glass fibers, carbon fibers, aromatic polyamide (Kevlar) fibers, silicon carbide fibers, poly(benzothiazole) and poly(benzimidazole) fibers, poly(benzooxazole) fibers, alumina, titania, quartz fibers, and the like. Selection of the fiber reinforcement type for these materials is determined by the performance requirements for the composite structure. For many spacecraft applications where high stiffness and low weight are critical, high modulus carbon or graphite type fibers are the preferred reinforcement. Examples of this type of fiber include P75, P100, P125 from Amoco, M40J, M55J, M60J from Toray, and K139c from Mitsubishi.
Alternatively, discontinuous, non-woven cloth, whiskers, chopped fiber and mat-type reinforcement materials may also be utilized.
Another method of making composite materials is by resin transfer molding (RTM). This is a process by which a resin system is transfered while at relatively low viscosity and under pressure into a closed mold with all of the important reinforcements and inserts already in place. The resin system can be prepared by premixing and placing the resin system into a resin injection pot or by metering components from separate pots at the appropriate mix ratio to an in-line static mixer or mixing zone. The resin system is then injected into the mold which is maintained under low pressure or under vacuum. The mold is often filled with resin while under vacuum to eliminate air from the mold space, to assist in resin injection and to aid in the removal of volatiles. The viscosity of the resin system dictates whether pot and/or mold heat is required. Low resin viscosity at the injection temperature is desirable to obtain best mold filling and mold wetting. After the mold is filled, it is sealed and heated in accordance with the appropriate cure schedule. The resulting molded part can then be removed from the mold and post-cured as necessary.
In order to achieve good fiber impregnation and low void content during RTM processing, resin viscosity below about 2000 cps at the injection temperature is highly desired, with resin viscosity below 1000 cps being preferred, and below 300 cps, most preferred. Further, the resin system must maintain this low viscosity for a period of time sufficient to completely fill the mold and impregnate the fiber preform. For RTM processing, such time is frequently measured in terms of the pot life of the resin, which can be defined as the time required for the resin to double its viscosity value. A resin pot life of at least 1 hour, and preferably two hours or more, is generally required for production of parts via RTM.
In another preferred embodiment of the invention, modified or unmodified resin systems as described are used in conjunction with woven fabric or non-woven mat reinforcements or preforms to directly produce advanced composite parts via processes such as RTM, RFI, VARTM. In these processes, the resin and fiber are combined during the actual part molding process. Any of the above listed fiber types may be utilized, with the most preferred type being determined by the performance characteristics of the application.
The following Examples are illustrative of the invention and are not intended to limit the scope thereof, which is defined by the appended claims.